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# The Apple Rule Implementations:
The packages `devtools/build/lib/rules/objc` and
`devtools/build/lib/rules/apple` implement the objc and ios Bazel rules.
## Interfacing from Skylark
Information exchange between skylark rules and native objc_* or ios_* rules
occurs by three mechanisms:
1) **`AppleToolchain:`**
`AppleToolchain.java` houses constants and static methods for use in rule
implementations. It is accessed in skylark through the global `apple_common`
namespace:
```
def __impl(ctx):
platform_dir = apple_common.apple_toolchain().platform_dir('iphoneos')
sdk_dir = apple_common.apple_toolchain().sdk_dir()
```
2) **`AppleConfiguration` and `ObjcConfiguration`**:
In Bazel, configuration fragments are used as containers for invocation-specific
build information (that is, information that cannot always be derived strictly
from BUILD files). The contents of these configurations can be inspected by
looking at `rules/objc/ObjcConfiguration.java` and
`rules/apple/AppleConfiguration.java`. To access a configuration fragment from
skylark, the fragment must be declared in the rule definition:
```
def __impl(ctx):
cpu = ctx.fragments.apple.ios_cpu()
my_rule = rule(
implementation = __impl
fragments = ['apple']
)
```
3) **`ObjcProvider`**:
The ObjcProvider maps "keys" to NestedSet instances, where "keys" are singleton
objects defined in ObjcProvider that identify a category of transitive
information to be communicated between targets in a dependency chain.
Native objc/ios rules export ObjcProvider instances, which are made available
to skylark dependants:
```
def __impl(ctx):
dep = ctx.attr.deps[0]
objc_provider = dep.objc
```
The provider can be queried by accessing fields that correspond to ObjcProvider
keys.
```
libraries = objc_provider.library # A SkylarkNestedSet of Artifacts
```
A skylark rule that is intended to be a dependency of native objc rules should
export an ObjcProvider itself. An ObjcProvider is constructed using a
constructor exposed on the apple_common namespace.
```
def __impl(ctx):
define = 'some_define'
objc_provider = apple_common.new_objc_provider(define=define)
return struct(objc = objc_provider)
```
Arguments to `new_objc_provider` should correspond to ObjcProvider keys, and
values should be skylark sets that should be added to the provider. Other
instances of ObjcProvider can also be used in provider construction.
```
def __impl(ctx):
dep = ctx.attr.deps[0]
define = 'some_define'
objc_provider = apple_common.new_objc_provider(providers=[dep.objc], define=define)
return struct(objc = objc_provider)
```
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